A transmitter lineup of a wireless communication device includes a power amplifier that has its output coupled to an antenna. Ideally, the input impedance of the antenna should match the output impedance of the power amplifier. However, the input impedance of the antenna is affected by external objects in its close vicinity, particularly metallic objects. This antenna characteristic may result in a significant impedance mismatch between the power amplifier and the antenna, under certain circumstances. Such an impedance mismatch causes radio frequency (RF) power to be reflected from the antenna input back into the power amplifier. This tends to reduce the power amplifier's linearity, RF output power, and efficiency.
Some devices include an isolator at the output of the power amplifier, which absorbs the reflected power, thus improving the linearity. However, an isolator is a relatively large and costly component, and its inclusion in a handheld device may undesirably add weight and size to the device. In addition, an isolator tends to limit the transmitted power under mismatch. Accordingly, isolators are not particularly desirable for use in handheld devices. Another technique for addressing impedance mismatch issues includes adjusting the drive level at the input of the power amplifier so that the output voltage swing can be reduced. However, this technique has the drawback of significantly reducing the transmitted RF output power.